Enteric flora expands gut lamina propria CX3CR1+ dendritic cells supporting inflammatory immune responses under normal and inflammatory conditions

Mucosal vaccination in a murine gnotobiotic model of Giardia lamblia infection

Giardia lamblia is an important protozoan cause of diarrheal disease worldwide, delayed development and cognitive impairment in children in low- and middle-income countries, and protracted post-infectious syndromes in developed regions. G. lamblia resides in the lumen and at the epithelial surface of the proximal small intestine but is not mucosa invasive. The protozoan parasite is genetically diverse with significant genome differences across strains and assemblages. Animal models, particularly murine models, have been instrumental in defining mechanisms of host defense against G. lamblia, but mice cannot be readily infected with most human pathogenic strains. Antibiotic pretreatment can increase susceptibility, suggesting that the normal microbiota plays a role in controlling G. lamblia infection in mice, but the broader implications on susceptibility to diverse strains are not known. Here, we have used gnotobiotic mice to demonstrate that robust intestinal infection can be achieved for a broad set of human-pathogenic strains of the genetic assemblages A and B. Furthermore, gnotobiotic mice were able to eradicate infection with a similar kinetics to conventional mice after trophozoite challenge. Germ-free mice could also be effectively immunized by the mucosal route with a protective antigen, α1-giardin, in a manner dependent on CD4 T cells. These results indicate that the gnotobiotic mouse model is powerful for investigating acquired host defenses in giardiasis, as the mice are broadly susceptible to diverse G. lamblia strains yet display no apparent defects in mucosal immunity needed for controlling and eradicating this lumen-dwelling pathogen.

A germ-free humanized mouse model shows the contribution of resident microbiota to human-specific pathogen infection

Germ-free (GF) mice, which are depleted of their resident microbiota, are the gold standard for exploring the role of the microbiome in health and disease; however, they are of limited value in the study of human-specific pathogens because they do not support their replication. Here, we develop GF mice systemically reconstituted with human immune cells and use them to evaluate the role of the resident microbiome in the acquisition, replication and pathogenesis of two human-specific pathogens, Epstein-Barr virus (EBV) and human immunodeficiency virus (HIV). Comparison with conventional (CV) humanized mice showed that resident microbiota enhance the establishment of EBV infection and EBV-induced tumorigenesis and increase mucosal HIV acquisition and replication. HIV RNA levels were higher in plasma and tissues of CV humanized mice compared with GF humanized mice. The frequency of CCR5+ CD4+ T cells throughout the intestine was also higher in CV humanized mice, indicating that resident microbiota govern levels of HIV target cells. Thus, resident microbiota promote the acquisition and pathogenesis of two clinically relevant human-specific pathogens.

NOD2 in monocytes negatively regulates macrophage development through TNFalpha

Objective

It is believed that intestinal recruitment of monocytes from Crohn's Disease (CD) patients who carry NOD2 risk alleles may repeatedly give rise to recruitment of pathogenic macrophages. We investigated an alternative possibility that NOD2 may rather inhibit their differentiation from intravasating monocytes.

Design

The monocyte fate decision was examined by using germ-free mice, mixed bone marrow chimeras and a culture system yielding macrophages and monocyte-derived dendritic cells (mo-DCs).

Results

We observed a decrease in the frequency of mo-DCs in the colon of Nod2-deficient mice, despite a similar abundance of monocytes. This decrease was independent of the changes in the gut microbiota and dysbiosis caused by Nod2 deficiency. Similarly, the pool of mo-DCs was poorly reconstituted in a Nod2-deficient mixed bone marrow (BM) chimera. The use of pharmacological inhibitors revealed that activation of NOD2 during monocyte-derived cell development, dominantly inhibits mTOR-mediated macrophage differentiation in a TNFα-dependent manner. These observations were supported by the identification of a TNFα-dependent response to muramyl dipeptide (MDP) that is specifically lost when CD14-expressing blood cells bear a frameshift mutation in NOD2.

Conclusion

NOD2 negatively regulates a macrophage developmental program through a feed-forward loop that could be exploited for overcoming resistance to anti-TNF therapy in CD.

Probiotic-educated Tregs are more potent than naïve Tregs for immune tolerance in stressed new-born mice

When new-born mice are subjected to acute maternal separation stress, cow-milk based formula feeding, and brief recurrent hypoxia with cold stress, they develop gut inflammation similar to the phenotype of neonatal necrotizing enterocolitis, characterised by an increase in gut mucosal effector T (Teffs) and reduced Foxp3⁺ regulatory T (Tregs) cells. The imbalance can be prevented by probiotic Limosilactobacillus reuteri DSM 17938 (LR 17938). We hypothesised that LR 17938 could potentiate a tolerogenic function of Tregs. To analyse whether LR 17938 can educate Tregs to improve their tolerogenic potency during neonatal stress, we isolated T cells (Tregs and Teffs) from 'donor' mice fed with either LR 17938 (10⁷ cfu) or control media. The cells were adoptively transferred (AT) by intraperitoneal injection (5 × 10⁵ cells/mouse) to new-born (d5) recipient mice. Mice were then separated from their dams, fed formula by gavage, and exposed to hypoxia and cold stress (NeoStress) for 4 days. We analysed the percentage of Tregs in CD4⁺T helper cells in the intestine (INT) and mesenteric lymph nodes (MLN) of recipient mice. We found that: (1) the percentage of Tregs in the INT and MLN following NeoStress were significantly reduced compared to dam-fed unstressed mice; (2) AT of either naïve Tregs or LR-educated Tregs to mice with Neostress increased the percentage of Tregs in the INT and MLN compared to the percentage in NeoStress mice without Treg treatment; however, LR-educated Tregs increased the Tregs significantly more than naïve Tregs; and (3) AT of LR-educated Tregs reduced pro-inflammatory CD44⁺Foxp3^-NonTregs and inflammatory CX3CR1⁺ dendritic cells in the intestinal mucosa of NeoStress mice. In conclusion, adoptive transfer of Tregs promotes the generation of and/or migration of endogenous Tregs in the intestinal mucosa of recipient mice. Importantly, probiotic-educated Tregs are more potent than naïve Tregs to enhance immune tolerance following neonatal stress.

Strategy and application of manipulating DCs chemotaxis in disease treatment and vaccine design

As the most versatile antigen-presenting cells (APCs), dendritic cells (DCs) function as the cardinal commanders in orchestrating innate and adaptive immunity for either eliciting protective immune responses against canceration and microbial invasion or maintaining immune homeostasis/tolerance. In fact, in physiological or pathological conditions, the diversified migratory patterns and exquisite chemotaxis of DCs, prominently manipulate their biological activities in both secondary lymphoid organs (SLOs) as well as homeostatic/inflammatory peripheral tissues in vivo. Thus, the inherent mechanisms or regulation strategies to modulate the directional migration of DCs even could be regarded as the crucial cartographers of the immune system. Herein, we systemically reviewed the existing mechanistic understandings and regulation measures of trafficking both endogenous DC subtypes and reinfused DCs vaccines towards either SLOs or inflammatory foci (including neoplastic lesions, infections, acute/chronic tissue inflammations, autoimmune diseases and graft sites). Furthermore, we briefly introduced the DCs-participated prophylactic and therapeutic clinical application against disparate diseases, and also provided insights into the future clinical immunotherapies development as well as the vaccines design associated with modulating DCs mobilization modes.

The effect of probiotics on immune responses and their therapeutic application: A new treatment option for multiple sclerosis

Multiple sclerosis (MS) is known as a chronic inflammatory disease (CID) that affects the central nervous system and leads to nerve demyelination. However, the exact cause of MS is unknown, but immune system regulation and inhibiting the function of inflammatory pathways may have a beneficial effect on controlling and improving the disease. Studies show that probiotics can alter the gut microbiome, thereby improving and affecting the immune system and inflammatory responses in patients with MS. The results show that probiotics have a good effect on the recovery of patients with MS in humans and animals. The present study investigated the effect of probiotics and possible therapeutic mechanisms of probiotics on immune cells and inflammatory cytokines. This review article showed that probiotics could improve immune cells and inflammatory cytokines in patients with MS and can play an effective role in disease management and control.

Mucosal vaccination: onward and upward

INTRODUCTION

The unique mucosal immune system allows the generation of robust protective immune responses at the front line of pathogen encounters. The needle-free delivery route and cold chain-free logistic requirements also provide additional advantages in ease and economy. However, the development of mucosal vaccines faces several challenges, and only a handful of mucosal vaccines are currently licensed. These vaccines are all in the form of live attenuated or inactivated whole organisms, whereas no subunit-based mucosal vaccine is available.

AREAS COVERED

The selection of antigen, delivery vehicle, route and adjuvants for mucosal vaccination are highly important. This is particularly crucial for subunit vaccines, as they often fail to elicit strong immune responses. Emerging research is providing new insights into the biological and immunological uniqueness of mucosal tissues. However, many aspects of the mucosal immunology still await to be investigated.

EXPERT OPINION

This article provides an overview of the current understanding of mucosal vaccination and discusses the remaining knowledge gaps. We emphasize that because of the potential benefits mucosal vaccines can bring from the biomedical, social and economic standpoints, the unmet goal to achieve mucosal vaccine success is worth the effort.

Biomarkers in oral immunotherapy

Food allergy (FA) is a global health problem that affects a large population, and thus effective treatment is highly desirable. Oral immunotherapy (OIT) has been showing reasonable efficacy and favorable safety in most FA subjects. Dependable biomarkers are needed for treatment assessment and outcome prediction during OIT. Several immunological indicators have been used as biomarkers in OIT, such as skin prick tests, basophil and mast cell reactivity, T cell and B cell responses, allergen-specific antibody levels, and cytokines. Other novel indicators also could be potential biomarkers. In this review, we discuss and assess the application of various immunological indicators as biomarkers for OIT.

Macrophages in the gut: Masters in multitasking

The gastrointestinal tract has the important task of absorbing nutrients, a complex process that requires an intact barrier allowing the passage of nutrients but that simultaneously protects the host against invading microorganisms. To maintain and regulate intestinal homeostasis, the gut is equipped with one of the largest populations of macrophages in the body. Here, we will discuss our current understanding of intestinal macrophage heterogeneity and describe their main functions in the different anatomical niches of the gut during steady state. In addition, their role in inflammatory conditions such as infection, inflammatory bowel disease, and postoperative ileus are discussed, highlighting the roles of macrophages in immune defense. To conclude, we describe the interaction between macrophages and the enteric nervous system during development and adulthood and highlight their contribution to neurodegeneration in the context of aging and diabetes.

Directly recruited GATA6 + peritoneal cavity macrophages contribute to the repair of intestinal serosal injury

Recruitment of bone marrow derived monocytes via bloodstream and their subsequent conversion to CX3CR1⁺ macrophages in response to intestinal injury is dependent on CCR2, Nr4a1, and the microbiome. This process is critical for proper tissue repair; however, GATA6⁺ peritoneal cavity macrophages might represent an alternative, more readily available source of mature and functional myeloid cells at the damaged intestinal locations. Here we show, using spinning-disk confocal microscopy, that large F4/80^hiGATA6⁺ peritoneal cavity macrophages promptly accumulate at damaged intestinal sites upon intestinal thermal injury and upon dextran sodium sulfate induced colitis in mice via a direct route from the peritoneal cavity. In contrast to bloodstream derived monocytes/macrophages, cavity macrophages do not depend on CCR2, Nr4a1 or the microbiome for recruitment, but rather on the ATP-release and exposed hyaluronan at the site of injury. They participate in the removal of necrotic cells, revascularization and collagen deposition and thus resolution of tissue damage. In summary, peritoneal cavity macrophages represent a rapid alternative route of intestinal tissue repair to traditional monocyte-derived macrophages.

Upon intestinal injury, bone marrow derived monocytes are recruited to the damaged site through the bloodstream. Authors here show that peritoneal cavity macrophages directly migrate to the damaged intestine in an ATP and hyaluronan dependent manner, and participate in the restoration of tissue integrity.

Advancedoral vaccine delivery strategies for improving the immunity

Infectious diseases continue to inflict a high global disease burden. The consensus is that vaccination is the most effective option against infectious diseases. Oral vaccines have unique advantages in the prevention of global pandemics due to their ease of use, high compliance, low cost, and the ability to induce both systemic and mucosal immune responses. However, challenges of adapting vaccines for oral administration remain significant. Foremost among these are enzymatic and pH-dependent degradation of antigens in the stomach and intestines, the low permeability of mucus barrier, the nonspecific uptake of antigens at the intestinal mucosal site, and the immune suppression result from the elusive immune tolerance mechanisms. Innovative delivery techniques promise great potential for improving the flexibility and efficiency of oral vaccines. A better understanding of the delivery approaches and the immunological mechanisms of oral vaccine delivery systems may provide new scientific insight and tools for developing the next-generation oral vaccine. Here, an overview of the advanced technologies in the field of oral vaccination is proposed, including mucus-penetrating nanoparticle (NP), mucoadhesive delivery vehicles, targeting antigen-presenting cell (APC) nanocarriers and enhanced paracellular delivery strategies and so on. Meanwhile, the mechanisms of delivery vectors interact with mucosal barriers are discussed.

A transepithelial pathway delivers succinate to macrophages, thus perpetuating their pro-inflammatory metabolic state

The gut metabolite composition determined by the microbiota has paramount impact on gastrointestinal physiology. However, the role that bacterial metabolites play in communicating with host cells during inflammatory diseases is poorly understood. Here, we aim to identify the microbiota-determined output of the pro-inflammatory metabolite, succinate, and to elucidate the pathways that control transepithelial succinate absorption and subsequent succinate delivery to macrophages. We show a significant increase of succinate uptake into pro-inflammatory macrophages, which is controlled by Na+-dependent succinate transporters in macrophages and epithelial cells. Furthermore, we find that fecal and serum succinate concentrations were markedly augmented in inflammatory bowel diseases (IBDs) and corresponded to changes in succinate-metabolizing gut bacteria. Together, our results describe a succinate production and transport pathway that controls the absorption of succinate generated by distinct gut bacteria and its delivery into macrophages. In IBD, this mechanism fails to protect against the succinate surge, which may result in chronic inflammation.

Role of Goblet Cells in Intestinal Barrier and Mucosal Immunity

Goblet cells and the mucus they secrete serve as an important barrier, preventing pathogens from invading the mucosa to cause intestinal inflammation. The perspective regarding goblet cells and mucus has changed, with current evidence suggesting that they are not passive but play a positive role in maintaining intestinal tract immunity and mucosal homeostasis. Goblet cells could obtain luminal antigens, presenting them to the underlying antigen-presenting cells (APCs) that induces adaptive immune responses. Various immunomodulatory factors can promote the differentiation and maturation of goblet cells, and the secretion of mucin. The abnormal proliferation and differentiation of goblet cells, as well as the deficiency synthesis and secretion of mucins, result in intestinal mucosal barrier dysfunction. This review provides an extensive outline of the signaling pathways that regulate goblet cell proliferation and differentiation and control mucins synthesis and secretion to elucidate how altering these pathways affects goblet functionality. Furthermore, the interaction between mucins and goblet cells in intestinal mucosal immunology is described. Therefore, the contribution of goblet cells and mucus in promoting gut defense and homeostasis is illustrated, while clarifying the regulatory mechanisms involved may allow the development of new therapeutic strategies for intestinal disorders.

Niche-specific functional heterogeneity of intestinal resident macrophages

Intestinal resident macrophages are at the front line of host defence at the mucosal barrier within the gastrointestinal tract and have long been known to play a crucial role in the response to food antigens and bacteria that are able to penetrate the mucosal barrier. However, recent advances in single-cell RNA sequencing technology have revealed that resident macrophages throughout the gut are functionally specialised to carry out specific roles in the niche they occupy, leading to an unprecedented understanding of the heterogeneity and potential biological functions of these cells. This review aims to integrate these novel findings with long-standing knowledge, to provide an updated overview on our understanding of macrophage function in the gastrointestinal tract and to speculate on the role of specialised subsets in the context of homoeostasis and disease.

How Changes in the Nutritional Landscape Shape Gut Immunometabolism

Cell survival, proliferation and function are energy-demanding processes, fuelled by different metabolic pathways. Immune cells like any other cells will adapt their energy production to their function with specific metabolic pathways characteristic of resting, inflammatory or anti-inflammatory cells. This concept of immunometabolism is revolutionising the field of immunology, opening the gates for novel therapeutic approaches aimed at altering immune responses through immune metabolic manipulations. The first part of this review will give an extensive overview on the metabolic pathways used by immune cells. Diet is a major source of energy, providing substrates to fuel these different metabolic pathways. Protein, lipid and carbohydrate composition as well as food additives can thus shape the immune response particularly in the gut, the first immune point of contact with food antigens and gastrointestinal tract pathogens. How diet composition might affect gut immunometabolism and its impact on diseases will also be discussed. Finally, the food ingested by the host is also a source of energy for the micro-organisms inhabiting the gut lumen particularly in the colon. The by-products released through the processing of specific nutrients by gut bacteria also influence immune cell activity and differentiation. How bacterial metabolites influence gut immunometabolism will be covered in the third part of this review. This notion of immunometabolism and immune function is recent and a deeper understanding of how lifestyle might influence gut immunometabolism is key to prevent or treat diseases.

Dynamic role of macrophage CX3CR1 expression in inflammatory bowel disease

Inflammatory bowel disease (IBD), consisting of ulcerative colitis (UC) and Crohn's disease (CD), is featured by overactive immune response and enduring course of unrestrained colitis. Genetic predisposition and environmental factors are fundamental in disease progression. Notably, microbiota dysregulation and its interaction with host mucosal barrier perplex disease phenotype. Under experimental setting, distinct mouse models are established to mimic human colitis process, including infection induced dysbiosis, dextran sulfate sodium (DSS) etc. induced barrier destruction, anti-CD40 L induced innate immunity dominant colitis and T cell transfer colitis model. Thus, from a more detailed aspect, IBD is heterogeneous and can be further classified into different subtypes based on the specific etiological pathways. As a typical inflammatory disorder, various immune cell types are involved in IBD pathogenesis. Among them, macrophages are believed to play a pivotal role. CX3CR1⁺ macrophages, deriving from peripheral patrolling CD14⁺ Ly6C^hi monocytes, are specified cell population dwelling in the gut. Accumulating evidence suggests that CX3CR1⁺ macrophages are critical for mucosal homeostasis and IBD pathogenesis, while some conflicts exist in current studies with both protective and harmful effects being revealed. Herein, we reviewed published literatures and found that the observed discrepancies stem from many aspects: the expression level of CX3CR1, the confounding dendritic cell subsets and most importantly, the different colitis stages and subtypes. Overall, CX3CR1 targeting strategy could be powerful weapon in fighting against colitis, but at the same time, the precise etiological and pathological mechanisms should be cautiously examined concerning the appropriate usage of CX3CR1 targeted therapy.

TL1A primed dendritic cells activation exacerbated chronic murine colitis

AIMS

Tumor necrosis factor-like ligand 1A (TL1A) has been proved to activate adaptive immunity in inflammatory bowel disease (IBD). However, its role in the regulation of intestinal dendritic cells (DCs) has not been fully characterized. This study aims to investigate the modulation of TL1A in DCs activation in murine colitis.

MATERIALS AND METHODS

Myeloid TL1A-Transgenic C57BL/6 mice and wild-type (WT) mice were administrated with dextran sulfate sodium (DSS) to explore the effects of TL1A in murine colitis. Bone marrow-derived DCs (BMDCs) were isolated to detect the ability of antigen phagocytosis and presentation. The expression of nuclear factor-κB (NF-κB) pathway and chemokines receptors (CCRs) was assessed by real-time PCR and Western blot.

KEY FINDINGS

Myeloid cells with constitutive TL1A expression developed worsened murine colitis with exacerbated T_H1/T_H17 cytokine responses. Intestinal DCs from TL1A transgenic mice expressed high levels of costimulatory molecules (CD80 and CD86) with increased pro-inflammatory cytokines of IL-1β, TNF-α and IL-12/23 p40. Mechanistic studies showed that TL1A enhanced the phagocytotic ability of BMDCs. Moreover, TL1A enhanced the capacity of antigen process and presentation in BMDCs. Besides, TL1A induced the phosphorylation of NF-κB(p65) and IκBα. Meanwhile, higher expression of CCR2, CCR5, CCR7, and CX3CR1 was observed both in vivo and in vitro.

SIGNIFICANCE

TL1A exacerbated DSS-induced chronic experimental colitis, probably through activation and migration of dendritic cells, and therefore increasing the secretion of pro-inflammatory cytokines.

The Need to Consider Context in the Evaluation of Anti-infectious and Immunomodulatory Effects of Vitamin A and its Derivatives

Vitamin A and its derivatives (retinoids) act as potent regulators in many aspects of mammalian reproduction, development, repair, and maintenance of differentiated tissue functioning. Unlike other vitamins, Vitamin A and retinoids, which have hormonal actions, present significant toxicity, which plays roles in clinically relevant situations, such as hypervitaminosis A and retinoic acid ("differentiation") syndrome. Although clinical presentation is conspicuous in states of insufficient or excessive Vitamin A and retinoid concentration, equally relevant effects on host resistance to specific infectious agents, and in the general maintenance of immune homeostasis, may go unnoticed, because their expression requires either pathogen exposure or the presence of inflammatory co-morbidities. There is a vast literature on the roles played by retinoids in the maintenance of a tolerogenic, noninflammatory environment in the gut mucosa, which is considered by many investigators representative of a general role played by retinoids as anti-inflammatory hormones elsewhere. However, in the gut mucosa itself, as well as in the bone marrow and inflammatory sites, context determines whether one observes an anti-inflammatory or proinflammatory action of retinoids. Both interactions between specialized cell populations, and interactions between retinoids and other classes of mediators/regulators, such as cytokines and glucocorticoid hormones, must be considered as important factors contributing to this overall context. We review evidence from recent studies on mucosal immunity, granulocyte biology and respiratory allergy models, highlighting the relevance of these variables as well as their possible contributions to the observed outcomes.

Neonatal gut microbiome and immunity

Early life is a critical time window for the neonatal gut to be progressively populated with different bacterial species that collectively promote gut maturation. A fully developed and healthy gut microbiome in neonates is an important driver for the development of other aspects of health. Unlike the relatively stable gut microbiome in adults, the developing gut microbiome in neonates exhibits higher plasticity and adaptability. This also underscores the unique window of opportunity for intervention or preventive measures to improve long-term health through modulations of the gut microbiome in early life. Better understanding of the neonatal gut microbiome - how it arises and how it impacts immune cell development - will help us appreciate the underpinnings of immune-related diseases. Here, we examine recent findings on the neonatal gut microbiome and discuss their implications for understanding this important driver of the maturation of the immune system and immunity against infections in early life.

Intestinal Macrophages at the Crossroad between Diet, Inflammation, and Cancer

Intestinal macrophages are key players in the regulation of the oral tolerance, controlling gut homeostasis by discriminating innocuous antigens from harmful pathogens. Diet exerts a significant impact on human health, influencing the composition of gut microbiota and the developing of several non-communicable diseases, including cancer. Nutrients and microbiota are able to modify the profile of intestinal macrophages, shaping their key function in the maintenance of the gut homeostasis. Intestinal disease often occurs as a breakdown of this balance: defects in monocyte-macrophage differentiation, wrong dietary habits, alteration of microbiota composition, and impairment in the resolution of inflammation may contribute to the development of intestinal chronic inflammation and colorectal cancer. Accordingly, dietary interventions and macrophage-targeted therapies are emerging as innovative tools for the treatment of several intestinal pathologies. In this review, we will describe the delicate balance between diet, microbiota and intestinal macrophages in homeostasis and how the perturbation of this equilibrium may lead to the occurrence of inflammatory conditions in the gut. The understanding of the molecular pathways and dietary factors regulating the activity of intestinal macrophages might result in the identification of innovative targets for the treatments of intestinal pathologies.

The CX3CL1/CX3CR1 axis is upregulated in chronic kidney disease and contributes to angiotensin II-induced migration of vascular smooth muscle cells

BACKGROUND

The role of the chemokine axis, CX3CL1/CX3CR1, in the development of cardiovascular diseases has been widely speculated. Angiotensin II (Ang II) is a pivotal factor promoting cardiovascular complications in patients with chronic kidney disease (CKD). Whether there is a link between the two in CKD remains unclear.

METHODS

The uremic mice were treated with losartan for 8 weeks, and the expression of aortic CX3CL1/CX3CR1 was detected. Cultured mouse aortic vascular smooth muscle cells (VSMCs) were stimulated with Ang II, and then CX3CR1 expression was assessed by western blot. After the targeted disruption of CX3CR1 by transfection with siRNA, the migration of VSMCs was detected by transwell assay. Finally, both the activation of Akt pathway and the expression of IL-6 were detected by western blot.

RESULTS

Losartan treatment reduced the upregulation of aortic CX3CL1/CX3CR1 expression in uremic mice. In vitro, Ang II significantly upregulated CX3CR1 expression in VSMCs. Targeted disruption of CX3CR1 attenuated Ang II-induced migration of VSMCs. In addition, the use of CX3CR1-siRNA suppressed Akt phosphorylation and IL-6 production in VSMCs stimulated by Ang II.

CONCLUSIONS

The aortic CX3CL1/CX3CR1 is upregulated by Ang II in CKD, and it contributes to Ang II-induced migration of VSMCs in vitro.

Dissecting the phenotypic and functional heterogeneity of mouse inflammatory osteoclasts by the expression of Cx3cr1

Bone destruction relies on interactions between bone and immune cells. Bone-resorbing osteoclasts (OCLs) were recently identified as innate immune cells activating T cells toward tolerance or inflammation. Thus, pathological bone destruction not only relies on increased osteoclast differentiation, but also on the presence of inflammatory OCLs (i-OCLs), part of which express Cx3cr1. Here, we investigated the contribution of mouse Cx3cr1⁺ and Cx3cr1^neg i-OCLs to bone loss. We showed that Cx3cr1⁺ and Cx3cr1^neg i-OCLs differ considerably in transcriptional and functional aspects. Cx3cr1^neg i-OCLs have a high ability to resorb bone and activate inflammatory CD4⁺ T cells. Although Cx3cr1⁺ i-OCLs are associated with inflammation, they resorb less and have in vitro an immune-suppressive effect on Cx3cr1^neg i-OCLs, mediated by PD-L1. Our results provide new insights into i-OCL heterogeneity. They also reveal that different i-OCL subsets may interact to regulate inflammation. This contributes to a better understanding and prevention of inflammatory bone destruction.

Loss of the branched-chain amino acid transporter CD98hc alters the development of colonic macrophages in mice

Comprehensive development is critical for gut macrophages being essential for the intestinal immune system. However, the underlying mechanisms of macrophage development in the colon remain elusive. To investigate the function of branched-chain amino acids in the development of gut macrophages, an inducible knock-out mouse model for the branched-chain amino acid transporter CD98hc in CX3CR1⁺ macrophages was generated. The relatively selective deletion of CD98hc in macrophage populations leads to attenuated severity of chemically-induced colitis that we assessed by clinical, endoscopic, and histological scoring. Single-cell RNA sequencing of colonic lamina propria macrophages revealed that conditional deletion of CD98hc alters the “monocyte waterfall”-development to MHC II⁺ macrophages. The change in the macrophage development after deletion of CD98hc is associated with increased apoptotic gene expression. Our results show that CD98hc deletion changes the development of colonic macrophages.

CD98hc in macrophages attenuates the severity of colitis. This change in the macrophage development is associated with increased expression of apoptotic genes, suggesting that CD98hc maintains the gut homeostasis by ensuring the development of gut macrophages.

Perivascular localization of macrophages in the intestinal mucosa is regulated by Nr4a1 and the microbiome

While the ontogeny and recruitment of the intestinal monocyte/macrophage lineage has been studied extensively, their precise localization and function has been overlooked. Here we show by imaging the murine small and large intestines in steady-state that intestinal CX3CR1⁺ macrophages form an interdigitated network intimately adherent to the entire mucosal lamina propria vasculature. The macrophages form contacts with each other, which are disrupted in the absence of microbiome, monocyte recruitment (Ccr2^−/−), or monocyte conversion (Nr4a1^−/−). In dysbiosis, gaps exist between the perivascular macrophages correlating with increased bacterial translocation from the lamina propria into the bloodstream. The recruitment of monocytes and conversion to macrophages during intestinal injury is also dependent upon CCR2, Nr4a1 and the microbiome. These findings demonstrate a relationship between microbiome and the maturation of lamina propria perivascular macrophages into a tight anatomical barrier that might function to prevent bacterial translocation. These cells are also critical for emergency vascular repair.

Lamina propria macrophages are at the frontline of defense against intestinal pathogens. Here the authors reveal that CCR2 and NR4A1-dependent CX3CR1+ macrophages form a dense network around the vessels in the lamina propria, and implicate this anatomical structure into prevention of systemic bacterial dissemination.

Microbiota in organ transplantation: An immunological and therapeutic conundrum?

The gastrointestinal (GI) tract microbiota is an environmental factor that regulates host immunity in allo-transplantation (allo-Tx). It is required for the development of resistance against pathogens and the stabilization of mucosa-associated lymphoid tissue. The gut-microbiota axis may also precipitate allograft rejection by producing metabolites that activate host cell-mediated and humoral immunity. Here, we discuss new insights into microbial immunomodulation, highlighting ongoing attempts to affect commensal colonization in an attempt to ameliorate allograft rejection cascade. Recent progress on the use of antibiotics to modulate GI microbiota diversity and innate-adaptive immune interface are discussed. Our focus on the microbiota's influence of endoplasmic reticulum (ER) stress and autophagy signaling through hepatic EP4/CHOP/LC3B platforms reveals a novel molecular pathway and potential biomarkers determining the progression of allo-Tx damage. Understanding and harnessing the potential of microbiome/bacteriophage therapies may offer safe and effective means for personalized treatment to reduce risks of infections and immunosuppression in allo-Tx.

Gut Microbiota and Immune Responses

The gut microbiota consists of a dynamic multispecies community living within a particular niche in a mutual synergy with the host organism. Recent findings have revealed roles for the gut microbiota in the modulation of host immunity and the development and progression of immune-mediated diseases. Besides, growing evidence supports the concept that some metabolites mainly originated from gut microbiota are linked to the immune regulation implicated in systemic inflammatory and autoimmune disorders. In this chapter, we describe the recent advances in our understanding of how host-microbiota interactions shape the immune system, how they affect the pathogenesis of immune-associated diseases and the impact of these mechanisms in the efficacy of disease therapy.

Microbial Colonization in Adulthood Shapes the Intestinal Macrophage Compartment

BACKGROUND AND AIMS

Contact with distinct microbiota early in life has been shown to educate the mucosal immune system, hence providing protection against immune-mediated diseases. However, the impact of early versus late colonization with regard to the development of the intestinal macrophage compartment has not been studied so far.

METHODS

Germ-free mice were colonized with specific-pathogen-free [SPF] microbiota at the age of 5 weeks. The ileal and colonic macrophage compartment were analysed by immunohistochemistry, flow cytometry, and RNA sequencing 1 and 5 weeks after colonization and in age-matched SPF mice, which had had contact with microbiota since birth. To evaluate the functional differences, dextran sulfate sodium [DSS]-induced colitis was induced, and barrier function analyses were undertaken.

RESULTS

Germ-free mice were characterized by an atrophied intestinal wall and a profoundly reduced number of ileal macrophages. Strikingly, morphological restoration of the intestine occurred within the first week after colonization. In contrast, ileal macrophages required 5 weeks for complete restoration, whereas colonic macrophages were numerically unaffected. However, following DSS exposure, the presence of microbiota was a prerequisite for colonic macrophage infiltration. One week after colonization, mild colonic inflammation was observed, paralleled by a reduced inflammatory response after DSS treatment, in comparison with SPF mice. This attenuated inflammation was paralleled by a lack of TNFα production of LPS-stimulated colonic macrophages from SPF and colonized mice, suggesting desensitization of colonized mice by the colonization itself.

CONCLUSIONS

This study provides the first data indicating that after colonization of adult mice, the numeric, phenotypic, and functional restoration of the macrophage compartment requires the presence of intestinal microbiota and is time dependent.

Commensal Microbiota Regulate Vertebrate Innate Immunity-Insights From the Zebrafish

Microbial communities populate the mucosal surfaces of all animals. Metazoans have co-evolved with these microorganisms, forming symbioses that affect the molecular and cellular underpinnings of animal physiology. These microorganisms, collectively referred to as the microbiota, are found on many distinct body sites (including the skin, nasal cavity, and urogenital tract), however the most densely colonized host tissue is the intestinal tract. Although spatially confined within the intestinal lumen, the microbiota and associated products shape the development and function of the host immune system. Studies comparing gnotobiotic animals devoid of any microbes (germ free) with counterparts colonized with selected microbial communities have demonstrated that commensal microorganisms are required for the proper development and function of the immune system at homeostasis and following infectious challenge or injury. Animal model systems have been essential for defining microbiota-dependent shifts in innate immune cell function and intestinal physiology during infection and disease. In particular, the zebrafish has emerged as a powerful vertebrate model organism with unparalleled capacity for in vivo imaging, a full complement of genetic approaches, and facile methods to experimentally manipulate microbial communities. Here we review key insights afforded by the zebrafish into the impact of microbiota on innate immunity, including evidence that the perception of and response to the microbiota is evolutionarily conserved. We also highlight opportunities to strengthen the zebrafish model system, and to gain new insights into microbiota-innate immune interactions that would be difficult to achieve in mammalian models.

The retinal environment induces microglia-like properties in recruited myeloid cells

Background

Microglia are essential to the development of the CNS and its homeostasis. Our prior findings suggested a niche model to describe the behaviors of retinal microglia. Here, we ask whether new myeloid cells recruited to the retina are constrained to resemble endogenous microglia morphologically and functionally.

Methods

Use of CD11c^DTR/GFP transgenic mouse allowed identification of two niches of retinal microglia distinguished by being GFP^lo or GFP^hi. We also used transgenic mice in which CX3CR1⁺ cells expressed YFP and were depletable following tamoxifen-induced expression of diphtheria toxin subunit A. We employed several ablation and injury stimulation protocols to examine the origin and fate of myeloid cells repopulating the retina. Analysis of retinal myeloid cells was done by microscopy, flow cytometry, and qRT-PCR.

Results

We found that the origin of new GFP^hi and GFP^lo myeloid cells in the retina of CD11c^DTR/GFP mice, whether recruited or local, depended on the ablation and stimulation protocols. Regardless of origin, new GFP^lo and GFP^hi retinal myeloid cells were CD45^medCD11b⁺Ly6G⁻Ly6C^loIba1⁺F4/80⁺, similar to endogenous microglia. Following tamoxifen-induced diphtheria toxin ablation, myeloid cell repopulation differed in the retina compared to the brain and optic nerve. Stimulation of replacement GFP^hi cells was substantially attenuated in repopulating retinas after tamoxifen-induced diphtheria toxin ablation compared to control or radiation-ablated mice. In radiation bone marrow chimeric mice, replacement GFP^hi myeloid cells from the circulation were slow to repopulate the retina unless stimulated by an optic nerve crush injury. However, once stimulated, recruited GFP^hi cells were found to concentrate on injured retinal ganglion cells and were morphologically similar to GFP^hi cells in non-ablated control CD11c^DTR/GFP mice.

Conclusions

The results support the idea that GFP^hi cells in the CD11c^DTR/GFP mouse, whether recruited or from resident microglia, mark a unique niche of activated retinal myeloid cells. We conclude that the retinal environment has a potent influence on the function, morphology, and proliferative capacity of new myeloid cells regardless of their origin, compelling them to be equivalent to the endogenous microglia.

Electronic supplementary material

The online version of this article (10.1186/s12974-019-1546-9) contains supplementary material, which is available to authorized users.

Intestinal Macromolecular Transport Supporting Adaptive Immunity

The gastrointestinal tract performs opposing functions of nutrient absorption, barrier maintenance, and the delivery of luminal substances for the appropriate induction of tolerogenic or protective adaptive immunity. The single-layer epithelium lining the gastrointestinal tract is central to each of these functions by facilitating the uptake and processing of nutrients, providing a physical and chemical barrier to potential pathogens, and delivering macromolecular substances to the immune system to initiate adaptive immune responses. Specific transport mechanisms allow nutrient uptake and the delivery of macromolecules to the immune system while maintaining the epithelial barrier. This review examines historical observations supporting macromolecular transport by the intestinal epithelium, recent insights into the transport of luminal macromolecules to promote adaptive immunity, and how this process is regulated to promote appropriate immune responses. Understanding how luminal macromolecules are delivered to the immune system and how this is regulated may provide insight into the pathophysiology of inflammatory diseases of the gastrointestinal tract and potential preventative or therapeutic strategies.

Cytokines and Soluble Receptors in Breast Milk as Enhancers of Oral Tolerance Development

The postpartum period is an important window during which environmental factors can shape the life-long health of the infant. This time period often coincides with substantial milk consumption either in the form of breast milk or from cow's milk sources, such as infant formulas. Although breast milk is the most beneficial source of nutrients for infants during the first 6 months after birth, its role in regulating food allergy development, through regulation of oral tolerance, is still controversial. Breast milk contains several factors that can impact mucosal immune function, including immune cells, antibodies, microbiota, oligosaccharides, cytokines, and soluble receptors. However, there is considerable variation in the assessed levels of cytokines and soluble receptors between studies and across the lactation period. Most of these cytokines and soluble receptors are absent, or only found in limited quantities, in commercial baby formulas. Differences in content of these pluripotent factors, which impact on both the mother and the neonate, could contribute to the controversy surrounding the role of breast milk regulating oral tolerance. This review highlights current knowledge about the importance of cytokines and soluble receptors in breast milk on the development of oral tolerance and tolerance-relateddisorders. Understanding the mechanisms by which such milk components might promote oral tolerance could aid in the development of improved strategies for allergy prevention.

Origin, Differentiation, and Function of Intestinal Macrophages

Macrophages are increasingly recognized as essential players in the maintenance of intestinal homeostasis and as key sentinels of the intestinal immune system. However, somewhat paradoxically, they are also implicated in chronic pathologies of the gastrointestinal tract, such as inflammatory bowel disease (IBD) and are therefore considered potential targets for novel therapies. In this review, we will discuss recent advances in our understanding of intestinal macrophage heterogeneity, their ontogeny and the potential factors that regulate their origin. We will describe how the local environment of the intestine imprints the phenotypic and functional identity of the macrophage compartment, and how this changes during intestinal inflammation and infection. Finally, we highlight key outstanding questions that should be the focus of future research.

Immunomodulatory effects of probiotics: Can they be used to treat allergies and autoimmune diseases?

As a person ages, physiological, immunological and gut microbiome changes collectively result in an array of chronic conditions. According to the 'hygiene hypothesis' the increasing prevalence of immune-mediated disorders may be related to intestinal dysbiosis, leading to immune dysfunction and associated conditions such as eczema, asthma, allergies and autoimmune diseases. Beneficial probiotic bacteria can be utilized by increasing their abundance within the gastrointestinal lumen, which in turn will modulate immune cells, such as, T helper (Th)-1, Th2, Th17, regulatory T (Treg) cells and B cells, which have direct relevance to human health and the pathogenesis of immune disorders. Here, we describe the cross-talk between probiotics and the gastrointestinal immune system, and their effects in relation to inflammatory bowel disease, multiple sclerosis, allergies and atopic dermatitis.

CX3CL1/CX3CR1 Axis Contributes to Angiotensin II-Induced Vascular Smooth Muscle Cell Proliferation and Inflammatory Cytokine Production

Angiotensin II (Ang II) dysregulation has been determined in many diseases. The CX3CL1/CX3CR1 axis, which has a key role in cardiovascular diseases, is involved in the proliferation and inflammatory cytokine production of vascular smooth muscle cells (VSMCs). In this study, we aim to explore whether Ang II has a role in the expression of CX3CL1/CX3CR1, thus contributing to the proliferation and pro-inflammatory status of VSMCs. Cultured mouse aortic VSMCs were stimulated with 100 nmol/L of Ang II, and the expression of CX3CR1 was assessed by western blot. The results demonstrated that Ang II significantly up-regulated CX3CR1 expression in VSMCs and induced the production of reactive oxygen species (ROS) and the phosphorylation of p38 MAPK. Inhibitors of NADPH oxidase, ROS, and AT1 receptor significantly reduced Ang II-induced CX3CR1 expression. Targeted disruption of CX3CR1 by transfection with siRNA significantly attenuated Ang II-induced VSMC proliferation as well as down-regulated the expression of proliferating cell nuclear antigen (PCNA). Furthermore, CX3CR1-siRNA suppressed the effect of Ang II on stimulating Akt phosphorylation. Besides, the use of CX3CR1-siRNA decreased inflammatory cytokine production induced by Ang II treatment. Our results indicate that Ang II up-regulates CX3CR1 expression in VSMCs via NADPH oxidase/ROS/p38 MAPK pathway and that CX3CL1/CX3CR1 axis contributes to the proliferative and pro-inflammatory effects of Ang II in VSMCs.

Metagenomic comparison of the rectal microbiota between rhesus macaques (Macaca mulatta) and cynomolgus macaques (Macaca fascicularis)

Rhesus macaques (Macaca mulatta) and cynomolgus macaques (Macaca fascicularis) are frequently used in establishing animal models for human diseases. To determine the differences in gut microbiota between these species, rectal swabs from 20 rhesus macaques and 21 cynomolgus macaques were collected, and the microbial composition was examined by deep sequencing of the 16S rRNA gene. We found that the rectal microbiota of cynomolgus macaques exhibited significantly higher alpha diversity than that of rhesus macaques, although the observed number of operational taxonomic units (OTUs) was almost the same. The dominant taxa at both the phylum and genus levels were similar between the two species, although the relative abundances of these dominant taxa were significantly different between them. Phylogenetic Investigation of Communities by Reconstruction of Unobserved States (PICRUSt) showed significant differences in the functional components between the microbiota of the two species, in particular the lipopolysaccharide (LPS) synthesis proteins. The above data indicated significant differences in microbial composition and function between these two closely related macaque species, which should be taken into consideration in the future selection of these animals for disease models.

CX3CR1+ Macrophages and CD8+ T Cells Control Intestinal IgA Production

Secretory IgA is a key host defense mechanism that controls the intestinal microbiota. We investigated the role of CD11c⁺CX₃CR1⁺CD64⁺ macrophages in IgA production in the intestine. Intestinal CX₃CR1⁺ macrophages directly induced IgA secretion by B cells. Ag delivery to lamina propria (LP) CX₃CR1⁺ macrophages specifically induced intestinal IgA production. The induction of IgA by CX₃CR1⁺ macrophages required BAFF, a proliferation-inducing ligand, and TNF-α, but was surprisingly independent of TLR-mediated microbial recognition and retinoic acid signaling. IgA secretion by CX₃CR1⁺ macrophages was enhanced by LP CD8⁺ T cells through the secretion of IL-9 and IL-13. CX₃CR1⁺ macrophages and CD8⁺ T cells induced IgA production by B cells independently of mesenteric lymph nodes and Peyer patches. Our data reveal a previously unrecognized cellular circuitry in which LP CX₃CR1⁺ macrophages, B cells, and CD8⁺ T cells coordinate the protective Ig secretion in the small intestine upon peripheral Ag delivery.

Communication Between the Microbiota and Mammalian Immunity

Mammalian immune systems evolved within a diverse world dominated by microbes, making interactions between these two life-forms inevitable. Adaptive immunity protects against microbes through antigen-specific responses. In classical studies, these responses were investigated in the context of pathogenicity; however, we now know that they have significant effects on our resident microbes. In turn, microbes employ an arsenal of mechanisms to influence development and specificity of host immunity. Understanding these complex reactions will be necessary to develop microbiota-based strategies to prevent or treat disease. Here we review the literature detailing the cross talk between resident microbes with a focus on the specificity of host responses and the microbial molecules that influence them.

CX3CR1+ mononuclear phagocytes control immunity to intestinal fungi

Intestinal fungi are an important component of the microbiota, and recent studies have unveiled their potential in modulating host immune homeostasis and inflammatory disease. Nonetheless, the mechanisms governing immunity to gut fungal communities (mycobiota) remain unknown. We identified CX3CR1⁺ mononuclear phagocytes (MNPs) as being essential for the initiation of innate and adaptive immune responses to intestinal fungi. CX3CR1⁺ MNPs express antifungal receptors and activate antifungal responses in a Syk-dependent manner. Genetic ablation of CX3CR1⁺ MNPs in mice led to changes in gut fungal communities and to severe colitis that was rescued by antifungal treatment. In Crohn's disease patients, a missense mutation in the gene encoding CX3CR1 was identified and found to be associated with impaired antifungal responses. These results unravel a role of CX3CR1⁺ MNPs in mediating interactions between intestinal mycobiota and host immunity at steady state and during inflammatory disease.

Lactobacillus brevis KB290 With Vitamin A Ameliorates Murine Intestinal Inflammation Associated With the Increase of CD11c+ Macrophage/CD103- Dendritic Cell Ratio

BACKGROUND

The ratio of colonic anti-inflammatory CD11c+ macrophages (MPs) to inflammatory CD103- dendritic cells (DCs) plays pivotal roles in intestinal inflammation. Little is known about how the ratio is regulated by lactic acid bacteria (LAB) and bifidobacteria (Bif). We investigated the contribution of LAB/Bif to this ratio.

METHODS

We established an in vitro experimental system using human myeloblastic KG-1 cells, which differentiate into CD11c+ MP-like (CD11c+ MPL) and CD103- DC-like (CD103- DCL) cells, and explored effective LAB/Bif strains. The selected strain's effect on the colonic CD11c+ MP/CD103- DC ratio and intestinal inflammation was examined in mice, and the strain's underlying mechanisms were investigated in vitro.

RESULTS

We screened 19 strains of LAB/Bif, and found that Lactobacillus brevis KB290 (KB290) increased the CD11c+ MPL/CD103- DCL cell ratio only in the presence of a vitamin A (VA) metabolite, retinoic acid (RA). Supplementation of KB290 with VA increased the CD11c+ MP/CD103- DC ratio in healthy mouse and prevented the disruption of the ratio during colitis. Supplementation of KB290 with pro-VA (β-carotene) also increased the ratio in healthy mouse and ameliorated the development of colitis. The ratio was increased by reduction of CD103- DCs (or CD103- DCL cells). Our in vitro data suggested that KB290 induced cell death in CD103- DCL cells in the presence of RA signaling.

CONCLUSIONS

Supplementation of KB290 with VA increases the colonic CD11c+ MP/CD103- DC ratio associated with the amelioration of murine colitis, suggesting a possible way to control intestinal inflammation by LAB.

E6130, a Novel CX3C Chemokine Receptor 1 (CX3CR1) Modulator, Attenuates Mucosal Inflammation and Reduces CX3CR1+ Leukocyte Trafficking in Mice with Colitis

The chemokine fractalkine (CX3C chemokine ligand 1; CX3CL1) and its receptor CX3CR1 are involved in the pathogenesis of several diseases, including inflammatory bowel diseases such as Crohn's disease and ulcerative colitis, rheumatoid arthritis, hepatitis, myositis, multiple sclerosis, renal ischemia, and atherosclerosis. There are no orally available agents that modulate the fractalkine/CX3CR1 axis. [(3S,4R)-1-[2-Chloro-6-(trifluoromethyl)benzyl]-3-{[1-(cyclohex-1-en-1-ylmethyl)piperidin-4-yl]carbamoyl}-4-methylpyrrolidin-3-yl]acetic acid (2S)-hydroxy(phenyl)acetate (E6130) is an orally available highly selective modulator of CX3CR1 that may be effective for treatment of inflammatory bowel disease. We found that E6130 inhibited the fractalkine-induced chemotaxis of human peripheral blood natural killer cells (IC₅₀ 4.9 nM), most likely via E6130-induced down-regulation of CX3CR1 on the cell surface. E6130 had agonistic activity via CX3CR1 with respect to guanosine 5'-3-O-(thio)triphosphate binding in CX3CR1-expressing Chinese hamster ovary K1 (CHO-K1) membrane and had no antagonistic activity. Orally administered E6130 ameliorated several inflammatory bowel disease-related parameters in a murine CD4⁺CD45RB^high T-cell-transfer colitis model and a murine oxazolone-induced colitis model. In the CD4⁺CD45RB^high T-cell transfer model, E6130 inhibited the migration of CX3CR1⁺ immune cells and decreased the number of these cells in the gut mucosal membrane. These results suggest that E6130 is a promising therapeutic agent for treatment of inflammatory bowel disease.

Intestinal macrophages in Peyer's patches, sacculus rotundus and appendix of Angora rabbit

The largest pool of macrophages in the body is harboured by the intestinal mucosa. As the principal phagocytic component of the immune system, macrophages are essential for maintaining mucosal homeostasis as they prevent commensal bacteria from adhering to mucosal epithelial cells. This study provides a RAM11 immunohistochemical and electron microscopic investigation of the existence, localization and distribution of intestinal macrophages in organized gut-associated lymphoid tissue (GALT), including Peyer's patches (PPs), the sacculus rotundus (SR) and the appendix, in the Angora rabbit. Although rabbit intestinal macrophages did not express the tissue macrophage marker macrosialin (CD68), they expressed RAM11. RAM11-positive intestinal macrophages were mostly localized to the subepithelial dome region, interfollicular area and germinal centres (GCs) of the GALT and the lamina propria or submucosa of the ileum and jejunum devoid of PPs and were also observed in the follicle-associated epithelium of PPs, but not in that of the SR and appendix. RAM11-positive macrophages containing engulfed apoptotic bodies were present in the GCs of the lymphoid follicles in the GALT. Electron microscopy further revealed multiple macrophages containing apoptotic bodies within the GCs of the follicles in the GALT. Some macrophage aggregations were observed in the GC and between the GC and the corona region of the follicles in the SR and appendix. Rabbit intestinal macrophages thus undertake both potent phagocytic activity and the efficient scavenging of apoptotic cells. Immunohistochemical data suggest that RAM11 can be reliably used for the determination of intestinal macrophages in the GALT of rabbits.

Tissue microenvironment dictates the fate and tumor-suppressive function of type 3 ILCs

Nussbaum et al. found that tumor suppression through innate lymphoid cells (ILCs) cannot be predicted solely based on the ILC phenotype and lineage but that their immune properties are shaped both by their ontogeny and by the tissue microenvironment they reside in.

Innate lymphoid cells (ILCs) have been classified into “functional subsets” according to their transcription factor and cytokine profiles. Although cytokines, such as IL-12 and IL-23, have been shown to shape plasticity of ILCs, little is known about how the tissue microenvironment influences the plasticity, phenotype, and function of these cells. Here, we show clearly demarcated tissue specifications of Rorc-dependent ILCs across lymphoid and nonlymphoid organs. Although intestinal Rorc fate map–positive (Rorc^fm+) ILCs show a clear ILC3 phenotype, lymphoid tissue–derived Rorc^fm+ ILCs acquire an natural killer (NK) cell/ILC1-like phenotype. By adoptively transferring Rorc^fm+ ILCs into recipient mice, we show that ILCs distribute among various organs and phenotypically adapt to the tissue environment they invade. When investigating their functional properties, we found that only lymphoid-tissue resident Rorc^fm+ ILCs can suppress tumor growth, whereas intestinal Rorc^fm− ILC1s or NK cells fail to inhibit tumor progression. We thus propose that the tissue microenvironment, combined with ontogeny, provides the specific function, whereas the phenotype is insufficient to predict the functional properties of ILCs.

Antibiotics promote the sampling of luminal antigens and bacteria via colonic goblet cell associated antigen passages

Bacterial translocation is defined as the passage of live bacteria from the gut lumen to distant sites. Gut commensal bacteria translocation has been attributed to 'leakiness', or 'barrier breach' of the intestinal epithelium, allowing live bacteria to cross an inappropriately permeable barrier and disseminate to distant sites. Alternatively, studies suggest dendritic cells directly capture luminal commensal bacteria and transport them to distant sites in the steady-state by extending dendrites between epithelial cells into the lumen. Recently we identified translocation of commensal gut bacteria following antibiotics was associated with the formation of goblet cell associated antigen passages (GAPs) in the colon and dependent upon goblet cells (GCs). The translocation of native gut commensal bacteria resulted in low-level inflammatory responses and potentiated mucosal damage in response to concurrent epithelial injury. Here we extend these observations and demonstrate properties of colonic GAPs and observations supporting their priority in the translocation of colonic commensal bacteria.

Diversity and functions of intestinal mononuclear phagocytes

The intestinal lamina propria (LP) contains a diverse array of mononuclear phagocyte (MNP) subsets, including conventional dendritic cells (cDC), monocytes and tissue-resident macrophages (mφ) that collectively play an essential role in mucosal homeostasis, infection and inflammation. In the current review we discuss the function of intestinal cDC and monocyte-derived MNP, highlighting how these subsets play several non-redundant roles in the regulation of intestinal immune responses. While much remains to be learnt, recent findings also underline how the various populations of MNP adapt to deal with the challenges specific to their environment. Understanding these processes should help target individual subsets for 'fine tuning' immunological responses within the intestine, a process that may be of relevance both for the treatment of inflammatory bowel disease (IBD) and for optimized vaccine design.

Microbiota induces tonic CCL2 systemic levels that control pDC trafficking in steady state

Plasmacytoid dendritic cells (pDCs) detect viruses initiating antiviral type I interferon responses. The microbiota is known to shape immune responses, but whether it influences pDC homeostasis and/or function is poorly understood. By comparing pDCs in germ-free and specific pathogen-free mice, we found that the microbiota supports homeostatic trafficking by eliciting constitutive levels of the chemokine CCL2 that engages CCR2. Mononuclear phagocytes were required for tonic CCL2 levels. CCL2 was particularly important for trafficking of a CCR2^hi subset of pDCs that produced proinflammatory cytokines and was prone to apoptosis. We further demonstrated that CCR2 was also essential for pDC migration during inflammation. Wild-type (WT):Ccr2^-/- mixed bone marrow chimeras revealed that CCR2 promotes pDC migration in a cell-intrinsic manner. Overall, we identify a novel role for the microbiota in shaping immunity, which includes induction of CCL2 levels that control homeostatic trafficking of pDCs.

Homeostatic Immunity and the Microbiota

The microbiota plays a fundamental role in the induction, education, and function of the host immune system. In return, the host immune system has evolved multiple means by which to maintain its symbiotic relationship with the microbiota. The maintenance of this dialogue allows the induction of protective responses to pathogens and the utilization of regulatory pathways involved in the sustained tolerance to innocuous antigens. The ability of microbes to set the immunological tone of tissues, both locally and systemically, requires tonic sensing of microbes and complex feedback loops between innate and adaptive components of the immune system. Here we review the dominant cellular mediators of these interactions and discuss emerging themes associated with our current understanding of the homeostatic immunological dialogue between the host and its microbiota.

Macrophages in gastrointestinal homeostasis and inflammation

Monocyte-derived mononuclear phagocytes, particularly macrophages, are crucial to maintain gastrointestinal homeostasis in the steady state but are also important for protection against certain pathogens. However, when uncontrolled, they can promote immunopathology. Broadly two subsets of macrophages can be considered to perform the vast array of functions to complete these complex tasks: resident macrophages that dominate in the healthy gut and inflammation-elicited (inflammatory) macrophages that derive from circulating monocytes infiltrating inflamed tissue. Here, we discuss the features of resident and inflammatory intestinal macrophages, complexities in identifying and defining these populations and the mechanisms involved in their differentiation. In particular, focus will be placed on describing their unique ontogeny as well as local gastrointestinal signals that instruct specialisation of resident macrophages in healthy tissue. We then explore the very different roles of inflammatory macrophages and describe new data suggesting that they may be educated not only by the gut microenvironment but also by signals they receive during development in the bone marrow. Given the high degree of plasticity of gut macrophages and their multifaceted roles in both healthy and inflamed tissue, understanding the mechanisms controlling their differentiation could inform development of improved therapies for inflammatory diseases such as inflammatory bowel disease (IBD).

The commensal microbiota exacerbate infectious colitis in stressor-exposed mice

Exposure to a prolonged restraint stressor disrupts the colonic microbiota community composition, and is associated with an elevated inflammatory response to colonic pathogen challenge. Since the stability of the microbiota has been implicated in the development and modulation of mucosal immune responses, we hypothesized that the disruptive effect of the stressor upon the microbiota composition directly contributed to the stressor-induced exacerbation of pathogen-induced colitis. In order to establish a causative role for stressor-induced changes in the microbiota, conventional mice were exposed to prolonged restraint to change the microbiota. Germfree mice were then colonized by microbiota from either stressor-exposed or non-stressed control mice. One day after colonization, mice were infected with the colonic pathogen, Citrobacter rodentium. At six days post-infection, mice that received microbiota from stressor-exposed animals had significant increases in colonic pathology and pro-inflammatory cytokine (e.g. IL-1β) and chemokine (e.g. CCL2) levels after C. rodentium infection in comparison with mice that received microbiota from non-stressed mice. 16S rRNA gene sequencing revealed that microbial communities from stressed mice did not have any detectable Bifidobacterium present, a stark contrast with the microbial communities from non-stressed mice, suggesting that stressor-induced alterations in commensal, immunomodulatory Bifidobacterium levels may predispose to an increased inflammatory response to pathogen challenge. This study demonstrates that the commensal microbiota directly contribute to excessive inflammatory responses to C. rodentium during stressor exposure, and may help to explain why gastrointestinal disorders are worsened during stressful experiences.